The present invention relates to a manufacturing method of a target for sputtering, and more particularly, to a manufacturing method of a target for sputtering which can form stably an oxide thin film with high permeability for a long time at high power.
With the increased integration of DRAM memory chips, many methods have been proposed to increase capacitance within a limited cell area. These methods can be classified into three types: (1) a methods for making a dielectric film thin, (2) methods for increasing an effective area of a capacitor, and (3) methods for using a substance with a high dielectric constant.
In the first of these three methods, if the dielectric film becomes a thin film which has a thickness of less than 100.ANG., application to device having large capacitance is difficult because the degree of reliability is lowered by Fowler-Nordheim current.
In the second method, the capacitor should be manufactured into a three-dimensional structure to increase the effective area of the capacitor, but this complicates the manufacturing process and increases the costs thereof.
Therefore, recently, the third method which attempts to manufacture a planar type capacitor by using a substance with high permeability (for example, oxides such as SrTiO.sub.3, BaSrTiO.sub.3, PZT or PLZT) as a dielectric film is being actively studied.
An oxide with high permeability is deposited by metal organic chemical vapor deposition (MOCVD) or by sputtering. However, the gaseous material which is required as a precursor of MOCVD method and the process of the MOCVD method have not yet been fully developed. Therefore the oxide with high permeability is mass-produced by the sputtering method which is easier than the MOCVD method.
There are two kinds of sputtering methods for obtaining the oxide with high permeability in the form of a thin film. The first is sputtering a metal target with alloy of each constituent metal by a direct current discharge method in a reactive atmosphere, for example, of argon (Ar) and oxygen (O.sub.2). The second is manufacturing an oxide target by sintering an oxide having a similar composition to an objective thin film, thereafter obtaining a thin film by a radio-frequency (RF) discharge method.
The former metal target has an advantage that the thin film can be deposited at high speed. However, since the metal target and the thin film obtained therefrom have very different composition from each other and activity of the metal (Ba, Sr, Ti, etc.) is very strong, it has a disadvantage in that it is difficult to manufacture a uniform alloy target.
On the other hand, though a small amount of reactive gas is additionally required, the latter oxide target has an advantage in that the composition between the oxide target and the thin film obtained therefrom is almost the same and it is not difficult to manufacture the target.
However, the oxide target should be sputtered by the RF method because the oxide target generally has high electrical resistance. Also, the surface temperature of a target is increased by an incident energy of Ar .sup.+ ions during the sputtering process; that is, the temperature of the target is considerably increased even at a low applied power during the sputtering process because an oxide or a ceramic target has a lower thermal conductivity than a metal target and therefore the cooling efficiency is degraded.
When the temperature of the target is increased by sputtering as described above, the target's volume is expanded by thermal expansion, thereby causing a tensile stress.
The oxide or ceramic target has a higher strength than a metal material against a compression stress but has a very poor tensile stress. Therefore, the oxide or ceramic target cannot be stably discharged for a long time at high applied power, as damage may occur even at a low applied power.
Therefore, to form an oxide thin film with high permeability without destroying the target, the target should be sputtered with a low applied power. This causes a poor deposition rate of the oxide thin film, which is a serious disadvantage in that it makes mass-production difficult.